Positron Emission Tomography Tracer Design of Targeted Synthetic Peptides via ¹⁸F-Sydnone Alkyne Cycloaddition

Chemically synthesized, small peptides that bind with high affinity and specificity to CD8-expressing (CD8+) tumor-infiltrating T cells, yet retain the desirable characteristics of small molecules, hold valuable potential for diagnostic molecular imaging of immune response. Here, we report the development of ¹⁸F-labeled peptides targeting human CD8α with nanomolar affinity via the strain-promoted sydnone-alkyne cycloaddition with 4-[¹⁸F]fluorophenyl sydnone. The ¹⁸F-sydnone is produced in one step, in high radiochemical yield, and the peptide labeling proceeds rapidly. A hydrophilic chemical linker results in a tracer with favorable pharmacokinetic properties and improved image contrast, as demonstrated by in vivo PET imaging studies.